When a hearing-impaired person seeks help in the form of a hearing aid, a process of evaluation, prescription, initial fitting and subsequent fine-tuning takes place, at the end of which it is hoped that the client is experiencing an optimal degree of benefit from the hearing aid concomitant with his/her personal circumstances (degree and type of hearing loss, listening needs, disposable income, etc.). Achievement of optimal benefit from a hearing aid fitting is dependent on many factors, not least of which is prescription of appropriate sound signal processing parameters according to which the hearing aid shall operate. Correct prescription of these parameters minimizes the need for subsequent fine-tuning adjustments and ensures that such fine-tuning as is necessary proceeds from a meaningful starting point.
It has long been accepted that different users are best served with different choices of sound signal processing in their hearing aids. First and foremost, the user's audiometric data (e.g. absolute threshold of hearing at various frequencies) are often used as input data to a procedure whereby appropriate choices of frequency response and compression parameters are prescribed. Additional diagnostic data which sometimes are utilised by such procedures define other aspects of the client's hearing loss, for example whether it is conductive or sensorineural in origin. Software modules used during the dispensing of hearing aids often contain such procedures in automated form.
It has also long been accepted that certain non-medical characteristics of the client should be considered when making choices concerned with which product features to include in the aid being dispensed. For example, clients with poor dexterity may do better with a hearing aid in which a user-operated volume control is superfluous.
The state of the art of prescribing appropriate signal processing in hearing aids is summarised in FIG. 1. Personal data in the optional categories of audiometry, audiological and otorhinolaryngological diagnosis, listening needs, special physical requirements serve as input to a decision-making process (manual or automatic) which contains rules for combining and processing the input data, and whose output is one or more recommendations about the signal processing characteristics to be implemented in the client's hearing aid.
It is known that people experience a reduction of cognitive capacity with age, which is thought to be due to the general effects of aging on the neurophysiological systems of the body. Cognitive deficits of this sort lead to poorer and slower performance in tasks requiring the interpretation and assimilation of information arriving from the environment. Included among such tasks is the understanding of speech. Recently it has been found that the optimum choice of signal processing in a hearing aid is not only dependent on the auditory characteristics of the client, but also on non-auditory characteristics, specifically the degree of generalised age-related reduction in cognitive capacity. Chronological age is not a perfect predictor of a person's level of cognitive function, but can be used as a simple indicator of high vs. low cognitive function, which is very reliable for the young and very old, and less reliable for people between about 60 and 80 years of age.
There are many medical conditions which are characterized by abnormal cognitive function in the sufferer, for example, Alzheimer's disease, dyslexia, schizophrenia, substance dependency, epilepsy. Although the causes of the abnormality differ widely, the effect in many cases is a general lowering of information-processing abilities similar to that seen with normal aging. A sub-group of conditions is characterized by abnormalities highly specific to the language processing or attention needed for effective speech understanding, for example, specific language impairment (SLI), central auditory processing deficit (CAPD). Thus, it is possible to use information about medical conditions other than hearing loss suffered by a hearing aid client to improve the decision-making process leading to recommendations affecting the signal processing to be implemented in the hearing aid.
It is also possible to identify categories of people whose cognitive processing capacity is likely to be unusually high, and thus make improved signal processing recommendations for these clients too. Such clients are typically characterised by having jobs which in themselves demand very high levels of cognitive processing, for example air traffic controllers, simultaneous interpreters, pilots.